From U.S. Pat. No. 3,755,144 a process is known in which a low octane hydrocarbon charge stock is isomerized to produce a branched hydrocarbon product. The process comprises isomerizing the charge stock in an isomerization zone and separating the effluent into a hydrogen-rich gaseous phase and hydrocarbon effluent. Hydrocarbon effluent containing 5 or 6 carbon atoms is passed to a molecular sieve separation zone, to provide a normal hydrocarbons stream and a branched hydrocarbons stream. The normal hydrocarbons stream is mixed with fresh charge stock and is again sent to the isomerization zone.
It has now surprisingly been found that by introducing feed into the process by adding fresh feed to the effluent of the isomerization step before separating off a hydrogen-containing gas, a better separation of hydrogen-containing gas and hydrocarbon effluent is obtained, with the same conventional separation means being used. A better separation makes that the volume of the hydrogen-containing gas is decreased, as the amount of hydrocarbons still present is decreased and therefore smaller units can be employed for further processing of the hydrogen-containing gas, resulting in lower operating costs.